I don't run a 603 but to trouble shoot
1st check for carbon build up in spark arrestor-a screen in the muffler. Clean and reinstall.
2nd Fuel cap vent - when it starts bogging loosen the fuel cap and see if that makes it recover-if yes, replace fuel cap.
3rd High speed fuel mixture-sound like it could be too lean
4th for $2 I'd replace the plug too
1st check for carbon build up in spark arrestor-a screen in the muffler. Clean and reinstall.
2nd Fuel cap vent - when it starts bogging loosen the fuel cap and see if that makes it recover-if yes, replace fuel cap.
3rd High speed fuel mixture-sound like it could be too lean
4th for $2 I'd replace the plug too